Note:

I understand that this may be working as intended, or may be something that won't be fixed, because it would probably require the entire lighting system for grass to be rewritten.
I did search for a report about this, but couldn't find one. Perhaps I searched incorrectly. :[

The bug

When a corner of grass extends past the edge of a block in both directions, it can stick through corners. This is intentional, based on the resolution of MC-63199.

The issue here, however, is that when this occurs, the parts of the grass that extend past the corner of the block retain the lighting of the origin block. This can cause a player to see a fully lit piece of grass even if the player is in a dark area, or a dark piece of grass even if the player is in a light area.

How to reproduce

_{NOTE: Using the exact coordinates is not necessary to reproduce the issue; this occurs at various coordinates throughout a world.}

1. Travel/teleport to the coordinates 1418 ~ -1660 and place a piece of grass
2. Stand on a block at 1419 ~ -1661 (at the same Y level as the grass) and surround yourself with two-high walls (not including the corners)
3. Cover the top of the room with a block (the room should be completely dark)
4. Run the command /time set noon
5. Observe that the extended part of the grass is lit, even though it exists in a dark space

Screenshots

Screenshot 1: The setup. An example of the setup used to reproduce, from steps 1 - 3.

Screenshot 2: Side view. An example of how the grass should extend through the blocks.

Screenshot 3: Player view. What you would see if you were to reproduce this issue.